Bulgaria, like Romania, is a fascinating place.
I stayed at the Hotel Ego in Plovdiv in 2015. I'd have preferred a place a bit closer to the historic core, but it was walkable for me. I don't remember the details, but the hotel was fine and relatively modern. I see that it gets a rating of 8.8 on booking.com these days.
I have no idea what your precise travel dates are, but I used the nights of April 26 and 27 to bring up the hotel list on booking.com, and I saw a lot of places with the "Last Available Room" warning. Even though we all know the hotels themselves may still hold inventory, I'm a bit surprised to see this for late April in Plovdiv. Just thought I'd warn you.
I liked Veliko Tarnovo a lot also. (Be prepared for spelling variations; in Bulgarian it's Trnovo, and people sort of choose their favorite vowel.) It was once the capital. There's historic architecture and a fortress. The tourist office has an audio guide you can rent. Like Plovdiv, V.T. does get its share of tourists. Both cities offer great shopping opportunities for folks who like crafts. Warning: I was pickpocketed in the shopping district of V.T. by two guys who blended into the background (not Roma, as I emphasized to the police).
In V.T. I stayed at the Meridian Hotel Bolyarski (though I think it has acquired the "Meridian" recently). It's also a relatively modern hotel, but it's built on the edge of the ravine with some of the floors below street level. My room didn't have much of a view (maybe none), which wasn't a big deal to me, but the Wi-Fi didn't work down on my floor, and there was a floor below mine that had no elevator service. Again, I'm seeing a lot of "Last Available Room" flags on booking.com. It's probably one of the nicest places in town. There was a small Asian tour group using it while I was there.
My notes indicate that the Bolyarski room included breakfast, but I have no memory of it whatsoever; I probably slept through it.
I also stayed in Burgas for a couple of nights so I could see Sozopol and Nesebar (aka Nesebr), two historic ports (now beachy tourist spots) on the Black Sea. Both towns have some very picturesque small churches, not all of them restored as of 2015. In Burgas I stayed in an apartment that was cheap but inconveniently located vis-a-vis transportation. Burgas itself wasn't terribly exciting, but it was interesting to be in a place that felt so off-the-tourist-grid.
For example, the casual restaurant near my apartment where I had dinner reeked of cigarette smoke to an unpleasant degree, though no one seemed to be smoking inside. I think the odor had gotten into the varnish on the wood furniture. You wouldn't find that in Plovdiv or Veliko Tarnovo.
It won't surprise you to know that there are more smokers in Bulgaria than in points to the west, and I suspect that your odds of finding a true non-smoking hotel room are better at the top of the economic ladder. Wasn't it Rick who said something to the effect that in Bulgaria you can find a non-smoking room, but the label doesn't mean no one has smoked in it; it means you are free not to smoke in it if you so choose? Both my hotels and the apartment were fine in that regard.
Can't help with Rila; unfortunately, I didn't have time to try to get there via public transportation.
As in Romania, you'll generally find young people who can converse in English.